Joseph Balicki is arrested by the Nazis. After a year, he escapes adn returns to Warsaw. His old house was destroyed. He finds out that his wife was arrested and that his children have never been found. In his old house, Joseph finds a boy sitting among the ruins with a black cat. The boy, Jan, has a letter opener called the Silver Sword. Jan helps Joseph find a train to go to Switzerland, where his wife has family.
The three children had escaped from the Nazis and managed to survive together. Edek managed to steal food and clothes for his sisters. He was captured by the Nazis. Ruth and Bronia manage to stay alive until Warsaw is liberated in 1944. They meet Jan and take him as a part of the family. A Russian sentry helps Ruth, and is able to find out that Edek escaped is in Posen. The three children go find him.
They all travel to Berlin. Edek, who has tuberculosis, and Jan are arrested for stealing food. Edek is cleard, but Jan spends a week in detention. A farmer then takes them in. A Burgomaster realizes the children are Polish and demand they be returned. The farmer helps them escape. After a scary boat trip during a storm, the children are reunited with their parents, who adopt Jan. They live happily in Switzerland.